Quantification of molecular oxygen and rates of oxygen uptake by biological and chemical samples is an important analytical task. Rates of oxygen uptake (or release) can serve as a biomarker of cell/organism viability, metabolic status, and alterations caused by endogenous and exogenous stimuli, such as drug/effector action, disease, or pathological processes. Also, many oxygen-dependent enzymatic and chemical reactions can be monitored via oxygen consumption, thus allowing quantification of corresponding enzymes, their substrates, products and modulators of activity (i.e. inhibitors or activators).
Oxygen consumption in liquid biological samples can be quantified by measuring pressure change in the headspace of samples placed in closed test-vials containing built-in pressure sensors (U.S. Pat. No. 5,232,839). This method appears to lack sensitivity and accuracy for some applications and to be time-consuming due to high capacity of sample headspace for oxygen and rapid back-diffusion of ambient/headspace oxygen into the sample. It also appears to be difficult to apply this method to very small samples.
U.S. Pat. No. 5,371,016 and U.S. Pat. No. 6,080,574 describe optical systems for measurement of sample sterility and microbial growth, which operate with vials/tubes with fluorescence-based oxygen sensors built-into vials to which samples are added, sealed and monitored.
WO98/15645 describes a microbiological method whereby a biological sample containing living micro-organisms is assessed by measuring gradients of the dissolved oxygen, using a solid-state luminescence-based oxygen sensor.
U.S. Pat. No. 5,882,922, describes a system for measurement of oxygen consumption in samples using wells containing solid-state oxygen sensor coating applied to the bottom of each well or soluble oxygen probes added to each sample.
EP1465730 describes a two-piece sealable micro-chamber device for measurement of oxygen consumption by small biological samples, particularly those containing cells.
Other important gaseous metabolites include carbon dioxide (CO2), which is one of the main products of metabolism released by living organisms. Ammonia is also produced in considerable amounts by certain micro-organisms and biological processes. Similarly to oxygen sensing, a number of optical chemical probes and sensors for measuring these analytes have been described.
The invention is directed towards providing improvements in measuring low levels of consumption or release of such gaseous metabolites, such as O2, but also CO2, or ammonia, in small biological or chemical samples.